Or, it's holding too much pressure- like the coolant is in a pressure cooker... or, there's too much coolant in the engine... or, the radiator is clogged.
I'm going to let someone else chime in here with a better answer for you, but in my case (similar to yours) I had a 59 year old original radiator that was too clogged up. When I tried to back flush it from the bottom end with an air nozzle wrapped up in a towel, it sprang an entire congregation of new leaks (that the clogs had been holding), so I ended up replacing it.
The thermostat (somebody correct me) in the pump controls the water flow, so be sure to check its function, as well.
